As described in fuller detail in U.S. Pat. No. 4,468,773, the extraordinary spatial and temporal coherence of some types of gas laser beams has enabled such lasers to find wide application in the field of metrology, and as an element in the control of various high precision laser systems. The ever increasing sophistication required by many current experiments often depends on the attainment of very high orders of frequency stability, both short and long term, in a reference laser. While it has long been known that gas lasers are capable of emitting light waves of truly phenomenal frequency stability, demonstrations of stabilities, such as 1 part in 10.sup.14 [S. N. Bagaev, A. S. Dychkov, and V. P. Chebotaev, Sov. Tech. Phys. Lett. 5(5), May 1979], have been limited to relatively short periods of the order of minutes under ideal laboratory conditions.
Of the many factors that effect the output frequency of a frequency-stabilized gas laser, even of the most sophisticated type, probably the most important is the effect of retroreflection and the way in which that reflection is altered in phase by temperature. [W. R. C. Rowley and D. C. Wilson, Appl. Optics 11, 475 (1975); K. Tanaka and T. Kurosawa, Japan. J. Appl. Phys. 15, 2271 (1976)].
Electronic servo control methods to achieve very high frequency stability in small relatively inexpensive lasers have already been described [U.S. Pat. No. 4,468,773]. The problem of retroreflection from the rear surface of the high reflector in an integral end mirror plasma tube can be eliminated by the incorporation of a small prism in the reflector's original construction, or by a prism which is cemented on after the laser tube has been manufactured. It is customary to apply a high quality anti-reflection coating to the low reflector output mirror to increase the power output and eliminate interference effects, and such a coating also effectively narrows down the beam profile by reducing the "out-of-focus" reflection from the curved surfaces of the output mirror. At first glance such an out-of-focus reflection from the coated exterior surface of the output mirror would not seem to have any resonant coupling to the laser cavity and, therefore, to have no effect on the frequency stability of the laser. Indeed, the effect on frequency stability of retroreflection from the external surface of laser output mirrors seems not to have been discussed at all in either the scientific or the patent literature.
At the levels of frequency stability now in some demand, however, one can expect that there will be some coupling to the plasma nearest the output mirror from at least the primary retroreflection originating at the external surface of the output mirror. A variation in the temperature of the output mirror, which affects the thickness of the mirror and therefore the phase of the retroreflection, also affects the frequency stability of the laser. This effect, in fact, has been experimentally verified b the inventor.